1. Field of the Invention
The present invention relates to pin grid array (PGA) zero insertion force (ZIF) connectors.
2. Description of the Related Art
Pin grid array zero insertion force connectors are well known in the electronics industry for connecting a large number of connector pins on a mating device with contacts in the PGA ZIF connector. Conventional PGA ZIF connectors have limits imposed on both the total number of contacts in the connector and the minimum pitch of the contacts imposed by the dimensional and structural integrity of plastic housings and plastic actuator components. For example, molded plastics exhibit "post-mold shrinkage," a phenomenon which causes a reduction in the dimensions of a molded part as the plastic hardens. Post-mold shrinkage can be anticipated and designed for; however, it is difficult to provide the accurate dimensional tolerances needed for PGA ZIF connectors having many thousands of contacts. As the size of the mold increases, problems with post-mold shrinkage become more acute, and, further, it becomes more difficult to assure that the molds are filled with plastic.
Dimensional accuracy in PGA ZIF connectors is important for two reasons. First, the positions of the contacts must correspond to a standard matrix which is identical to the matrix of the connector pins to be inserted into the contacts. Disparities in positions can cause an inability to mate the contacts with the pins and may result in poor electrical connections between the contacts and pins.
Second, the actuator which opens or closes the contacts or forcibly engages the pins with the contacts must not introduce large variations in the contact force, which would create poor contacts at very low or marginal contact pressure or damage to contacts due to excessive contact pressures.
The limitations of conventional PGA ZIF connectors are exemplified by reference to several conventional designs. A first conventional design for a PGA ZIF connector is disclosed in "A ZIF Connector For Pin Grid Arrays," Harvey R. Waltersdorf, Electronic Products, Oct. 24, 1983. This design cannot be used with more than 1,000 contacts, at least in part due to a lack of dimensional and structural stability of its molded plastic actuator components. The normally closed contacts of this design are opened by sliding an actuator in a horizontal plane which causes a cam to slide up ramp-risers thereby moving the cam in a vertical plane to open the contacts. The forces necessary to operate this system increase dramatically as the size of the connector increases--particularly since the actuator and the cam are frictionally engaged; these actuation forces place great stresses on the actuator.
A second design proposed by ITT Cannon, "Zero Force Matched Impedance Pin Array Interconnecting System," Seventeenth Annual Connectors and Interconnection Technology Symposium Proceedings, Sept. 19-21, 1984, is indicated to be useful with 1000 connectors in a 4.times.4 inch array. The single-tine contacts of the ITT cannon design provide only a single point of contact with each mating device connector pin, and thus have a lack of redundancy which may be a critical requirement in high reliability applications. Electrical concretions between the contacts and connector pins are made by providing a force which deforms the contacts to engage the contacts with the connector pins. Contact beam length is critical to device performance because the spring force (contact force) of each contact varies with the cube of the beam length. Beam length is defined as the distance between the point at which the contact is supported by a printed circuit board and the contact point. The contacts are mounted in holes in the circuit board with solder, and thus beam length is directly related to the volume of solder in the hole in the circuit board. Since the amount of solder utilized per contact is difficult to control, variations in the beam length are unavoidable. Further, solder is not a good structural material due to its inherent malleability. In addition, the contacts are all simultaneously deformed by an actuator frame. Difficulties in maintaining dimensional tolerances in the actuator frame create large variations in contact force at different pin positions through the connector.
A PGA connector having 1800 pins has been proposed by IBM. The IBM PGA connector design is not a true ZIF connector since all of the contacts are simultaneously slidably engaged with connector pins during insertion of the connector pins into the PGA connector. See "Development of Interconnection Technology for Large Scale Integrated Circuits," IBM Journal R&D Vol. 26, No. 3, May, 1982, R. Babuka et al. This concept is not practical for large PGA devices because contact pressure is compromised by the need to reduce the total module engagement force. The sliding engagement of the contacts and connector pins requires actuating forces which cannot feasibly be provided when this design is used with a large number of contacts. In addition, the sliding engagement of the contacts and connector pins creates serious reliability problems due to susceptibility to damage of the small-diameter connector pins needed for high pin densities.
In summary, existing designs of ZIF connectors are not applicable for use with significantly higher contact counts due to the following factors: (1) problems with post-mold shrinkage and warp, and mold filling for larger molds; (2) the impacts of component and assembly tolerances on contact force and device stresses; (3) excessive actuating loads--especially those encountered when all contacts are actuated simultaneously by a single plastic actuator; (4) the complexity of the actuators; and (5) the inability to utilize a continuous pin field when the connector is segmented in order to divide the high actuating forces. In particular, it is not possible to produce large, e.g., 8-10 in.sup.2, connectors with a high number of pins, e.g., 10,000, having a pitch of less than 100 mils which are essential if a single connector is to be provided for a large chip carrier substrate holding, for example, 144 chips in a 12.times.12 array.